Neutralizing device



Dec. 12, 1933. w. J. 'KIMMELL 1,939,059

NEUTRALI Z ING DEVI CE vFiled March 20, 1930 INVENTOR Nil/[am J/fimme/l.

( Y dA/i/ A'TToRNEY Patented Dec. 12, 1933 UNITED, sTATE s PATENT OFFICEv 3 Claims. (01. 25o 27) This invention relates to neutralizing devicesand especially to such devices for neutralizing a substantial part ofthe hum produced by the alternating current apparatus containingthermionic tubes. i v

An object of the invention is to apply means to.

the audio stage of a radio receiving-set for neutralizing a substantialportion of the doublei'requency hum produced by the alternating-currentpower supplied to such set. More specifically, it is an object of theinvention to establish an alternating magnetic field in the immediateneighborhood of" one of the audio tubes of the set to produce a periodicchange in'the velocity of the electrons to the anode of'the tube and tothus neutralize, to a'large extent, the double frequency component ofthe hum inherent in'the t; ".1 I Y The single figure of the drawing is'adiagram of apparatus circuits embodying my invention. a

It has heretofore beencustomary to carefully so design andm'anufactur'ethe'various elements,

of an alternating-current radio receiving: set that the hum produced bythe useof alternating current will be reduced as much as possible; The

design of such elements has'not always'ben suc-v cessful in reducing thehum to the desired minimum and, furthermore'the additional care requiredin manufacturing the "set has added considrably to the cost of producingit. Instead of carefully designingeach portion of the'set to reduce thehum'to a'mi'nimu'm, it'is one of the purposes of this invention to applya neutralizing device to one of the stagesof the set to neutralize asubstantial portion of the hum produced by the several elements of theset. As the'faudio stage is generally the last stage of the set, itispreferred to apply this neutralizing device to one of-the tubes ofthe-audiostage; 1, This neutralizing device preferably comprises amagnetic means for producing an alternating magnetic field in theneighborhood of 'this'audio tube which means is preferablya coilsurrounding the tube. The field'of this coil makes a periodic'change invelocity of theelectrons and,

it formed in proper phase, will neutralize a substantial portion 'offthenormally inherent humproducing component inthe output circuit.

In *the drawing is disclosed variouselements common to a radio receivingset althoughit is not intended to limit. the invention to theparticularnum'ber; type, arrangement or purpose of theelemen'ts sodisclosed. v

The circuit diagram contains the antenna .10

forreceiving-the radio signal and also a radio amplifier 11, detectortube 12 and audio amplifier 13. If desired, the number of radio or audiotubes may be increased. However, thcsethree tubes constitute the basic.and well known stages of a radio receiving set. Other elements of thediagram, such as the transformer 14, condenser 15 and ground 16, are toowell known to need detailed description.

Connections 1'7 and 18, by which the apparatus may be attached to analternating current supply, such as the usual 110-volt commercialcircuit, are installed at a convenient place in the apparatus. Atransformer 19 has its primary coils attached to these connections 17and 18 and has its. secondary coils arranged to supply the various powerneeds of the apparatus. The secondary coil 20 of the transformer 19supplies current for'heating the-filaments 21, 22 and 23 of the varioustubes. .Another group of secondary coils of the, transformer 19 lead toa full-wave rectifier such as that composed of the two rectifyingtubes25 and 26 in conjunction with a wave filterincluding inductances 27and condensers 28.. Various taps 29, 30, 31, 32, and 33 along the'80resistance 34 across the output of, the rectifier permit the connectionsthereto for the various B and C voltages required in the operation ofthe apparatus. The output of the set is across the terminals 35 and 36to which aloud speaker is usually connected.

In my copending application filed jointly with Lee Sutherlin asco-inventor Serial No. 210,262; on August 3', 1927, are described thevarious causes that contribute to the production of a hum-producingcurrent in an alternatingecurrent-operated set. One of the mosttroublesome of these causes is the so-called electrostatic orthree-halves-power double-frequency current produced in the plate bythe, efi'ect of the potential drop along the alternating -current-heatedfilament.

Take for example, the filament 22 in the tube 12, although other tubesof the set could also be used as illustrations. Alternating currentpasses through this filament between the ends 40 and 41. The voltage isconstantly changing from a maximum to zero and then to a maximum of theopposite sign. But, during the cycle, one part of the filament may be ofzero voltage and other parts of maximum voltage of opposite signs. Theheating effect'of this current causes the filament to be in a stateadapted to give oif electrons," and the Bvoltage applied between thefilament and the plate 42 will drive these electrons across the spaceunder'the control of the is greater than at the mid point. At the same jCurrent instant, the end 41 has a section at the positive maximumpotential, with the result that the electron-driving voltage is less atthispoint than at the center of the filament. If there was a linearrelation between the electron-driving voltage and the current in theanode or plate, the greater potential at one end of the filament mightexactly compensate for the lesser potential at the other end. As therelation is of the order of the three-halves power, there is not thiscompensation, but a difference that produces surges in the anode circuitwhen first one end and then the other end of the filament reaches anegative maximum potential. As this phenomena occurs twice during eachcycle, a double-frequency component is produced in the plate circuitwhich will produce hum in the loud speaker.

Likewise, the other tubes will also. have this electrostaticdouble-frequency alternating-current component in their plates, with theresult that the final audio stage will very likely have, in its platecircuit, the resultant of this doublefrequency alternating-currentcomponent of the various tubes of the set.

A mathematical derivation of this alternatingcurrent double-frequencycomponent is as follows: The current flowing to the anode from thecathode may be calculated by the use of the well known Langmuir formulaI i,,=1v1e* where i is current from a hot filament of length 1 to aradio tube anode, e is the potentialdifferr ence between the cathode andthe anode and N is proportional to the number of electrons availableatthe cathode. I v I .In the "case of a three-electrode tube the Lang- 1muir formula takes the form v V where'E is anode voltage; Eg is gridvoltage; and is the amplification'constant'of the tube. Applying theforegoingformula to current between filament and anode, the potentialdifferencebetween the anode and anypoint on the cathode at a distance xfrom the mid point thereof is represented by e=e +i E sin pt E sin ptFdV Substituting for N the value as given above and dropping the terms ofpowers higher than the second of gives Upon integration, this gives forthe total anode This shows that the anode current consists of directcurrent component LE KE 16(e 4(e,,)%L

and an alternating current component LE KE No(e )i m-m)cos 2 pt which isof double frequency. I

As the usual commercial supply is of 60 cycles this alternating-currentcomponent will be of l20-cycle frequency.

Another hum-producing cause is the effect of the alternating magneticfield set up by the alternating current passing through the filament.This field deflects the electrons from the straight path that they wouldotherwise follow between the cathode or filament and the anode or plate.During each cycle of the alternating current, a

magnetic field of one sign, as, for instance, positive will be built upto a maximum and then decreased to zero and built up into a magneticfield in the other direction. Accordingly, the changing magnetic fieldwill-affect the passage of the electrons twice during each cycle withthe result that a double-frequency hum-producing component will also bepresent in the cathode circuit due to this cause. This so-calledmagnetic hum, however, isgenerally opposite to the electrostatic effect.7

Athird cause of hum-producing current results from the operation of therectifier. The wave filter attached to the rectifying tubes has to bevery carefully designed and manufactured to produce a constant voltage.If inexpensive materialsare used there is apt to be a doublefrequencyalternating-current component also produced in the set generally inphase with the electrostatic component. The alternating ourrent suppliedto thetransformer 19 changes in 1 direction twice during each cycle.This alternating voltage is theoretically changed by the rectifier to aconstant direct-current voltage but, in case the reotifierelements arenot exactly de signed, this direct-current voltage will be slightlypulsated at each maximum of the alternating current. This pulsation willbe of the order of the double frequency of the alternating current dueto the fact that the rectified current will have an increment of voltageduring each change in direction of alternating current. 'As thischangein direction occurs twice during each cycle there will be a pulsatingcomponent of double the frequency.

Accordingly, it has been discovered that an alternating-currentreceiving set has a hum mainly produced by a component in the platecircuit of double the frequency of the alternating current supplied tothe set for power purposes.

It is the purpose of this invention to provide means for neutralizingthe major portion of this hum-producing component in the'cir'cuit of theset. It is desirable to apply this neutralizing device to an audio stageof the set in order to neutralize as much as possible of the accumulatedhum-producing component. A preferred form of the device is disclosed inthe circuit drawing by the elements 46, 4'1 and 48.

It is desired to produce an alternating field in the immediateneighborhood of one of the audio tubes of the set to produce a periodicchange in the velocity of the electrons travelling to the plate and thusneutralize the major portion of the double-frequency variation normallyproduced in the plate circuit of the tube 13. This alternating field ispreferably produced by a coil 46 surrounding one of the audio tubes ofthe set. The phase and magnitude of this alternatingv field can beregulated by a potentiometer 4'7 and a variable-voltage iron-coretransformer 48 connected to the alternating-current supply andpreferably to the same circuit supplying the heating current for thetubes.

The voltage across the secondary of the transformer 48 is approximately90 out of phase with the voltage drop across the resistance 47. A propercombination of these voltages enables one to choose the desirable phaserelation of the current flowing through the neutralizing coil 46 toneutralize the hum-current component in the tube.

By way of example, and not to be considered limited thereto, the coil 46is one of the tests may be employed otherwise than that shown.

In some cases, it might be advantageous to apply the field to one of theaudio tubes preceding the power-output tube if more than one tube shouldbe used in the audio stage.

It is also possible that better rssuns might be" obtained, in somecases, by designing the set to produce a larger double-frequencyalternating component than is now the case in order that it may be morecompletely neutralized by the neutralizing device applied to the lastaudio stage. It is apparent that, with this neutralizing device, thevarious parts of the set do not have to be so carefully designedandassembled or made out of such expensive material, to minimize thisdouble-frequency alternating component because this component will beneutralized before the output is reached.

It is also apparent that the invention, although described as applied toa radio receiving set, is not limited to such application but may alsobe applied to other apparatus using tube structure in which a hum isproduced by alternating cur.- rent.

Although I have shown and described certain specific embodiments of myinvention, I am fully aware that many modifications thereof arepossible. My invention, therefore, is not to be restricted exceptinsofar as is necessitated by the prior art and by the spirit of theappended claims.

I claim is my invention:

1. An apparatus having a plurality of electron tubes,alternating-current connections for supplying powerto the cathodes ofsaid tubes, and means for neutralizing a substantial portion of the humnormally produced in the output supply of said apparatus by thealternating current, said means including a coil surrounding one of saidtubes.

2. An apparatus having a plurality of tubes, an anode and a. cathode insaid tubes, alternatingcurrent 1 connections for supplying heatingcurrent to the cathodes of said tubes, and a coil connected to saidalternating current connections surrounding one of said tubesforneutralizing substantially the major portion of the double frequency humproduced in said tube by the use of alternating current.

3. An apparatus having a plurality of tubes, an anode and a cathode insaid tubes, an altemating-current source for supplying heating currentto the cathodes of said tubes, and conductors connected to saidalternating-current source positioned outside oneof said tubes to set upa magnetic field which crossesthe space between neutralizingsubstantially the major portion of the double-frequency hum produced insaid tube by the use of alternating current.

WILLIAM J. KIMMELL.

